Indian J.Sci.Res. 14 (1): 284-289, 2017 ISSN: 0976-2876 (Print)

ISSN: 2250-0138 (Online)

1Corresponding author

HIGH TEMPERATURE MEASUREMENT USING FIBER BRAGG GRATING SENSOR

S. V. SHRUTHIa1 AND S. SUNDARAVADIVELU

b

aPG Scholar, Department of ECE, SSN College of Engineering, Chennai, India bProfessor, Department of ECE, SSN College of Engineering, Chennai, India

ABSTRACT

Measuring the temperature in high temperature industrial application is an important factor and a challenging part

where sensors need to withstand high temperature without destruction. Temperature measurement and monitoring in the

industries are necessary to ensure correct and accurate operation of the equipment. In these high temperature application

conventional electronic sensors like thermocouple, bimetal switches etc cannot withstand high temperature, malfunction due to

overheating and also easily pick up Electromagnetic Interference (EMI). In this paper these conventional sensors are replaced

by Fiber Bragg Grating (FBG) sensors which is based on the principle of measurement of reflected Bragg’s wavelength and the

corresponding shift in the wavelength for the temperature sensed. Advantages of FBG sensors for temperature measurement is

that its light weight, small size, flexibility, non interfering, low loss, long range sensing(remote sensing), multiplexing

capabilities, withstands high temperature. In this paper the simulation of 2D and 3D model is done using the Comsol software

and also the experiment is performed for FBG temperature sensors to depict the shift in Bragg’s wavelength that can be used

in high temperature monitoring in oil wells, high temperature optical sensing in gas turbines, widely used in nuclear reactors

which has elevated temperature and high levels of electromagnetic interference (EMI).

KEYWORDS: Electromagnetic Interference, Comsol Software, Bragg’s Wavelength, Fiber Bragg Grating Sensor.

Optical fiber based sensing technology have

several inherent advantages that make them attractive for

a wide range of industrial sensing applications. The

conventional sensors like thermocouples, resistance

temperature detectors, thermistors cannot be used in high

temperature measurement owing to failure at high

temperature, EMI coupling, and not appropriate to be

used in high microwave fields[Rao, et.al., 1997]. The

FBG sensors are typically small in size, passive, immune

to EMI, resistant to harsh environments and have a

capability to perform distributed sensing.

The Fiber Bragg Grating is optical fiber sensor

that is created by photo inscribing the core of the silica

fiber with the Ultra violet rays by the photomask

method. There is a periodic perturbations created in the

core refractive index of the optical fiber created by

exposure to intense UV radiation. The refractive index

of the fiber is permanently altered according to the

intensity of light it is exposed. The alteration in the

refractive index depends on the photosensitivity of the

fiber. The resulting periodic variation in the refractive

index is called a fiber Bragg grating. The Fig. 1 shows

the expanded view of FBG with the core of the fiber

being inscribed and grating is produced with the period

Λ.

The FBG sensors are made of germanium

doped silica because of its high melting point 938.2 °C

and 1600 °C respectively to withstand high temperature.

The FBG reflects a particular wavelength called the

Bragg’s wavelength and transmits all other wavelength

in a broadband source. This property of reflecting the

Bragg’s wavelength paves way for sensing applications

using FBG. The reflected Bragg’s wavelength is given as

λb=2 × n × Λ (1)

Figure 1: An expanded view of Fiber Bragg Grating

sensors.

In the equation (1) λb indicates the Braggs

wavelength and n for refractive index and Λ is the

grating period. The sensing function of an FBG

originates from the sensitivity of both the refractive

index of the optical fiber and the grating period within

the fiber to externally applied mechanical or thermal

perturbations. When a external physical parameter like

temperature is applied to the FBG there is a shift in the

reflected Bragg’s wavelength[Rao, et.al., 1997]. Based

on the shift the amount of variation in the temperature is

obtained. In this way the temperature can be monitored

and measured regularly in high temperature sensitive

regions. The FBG can measure high temperature as 1000

degree Celsius due to the characteristics of the fiber

material[Rao, et.al., 1997]. The shift in the wavelength

can be measured from the following equation:

∆λb =λb (1+ξ )∆T (2)

SHRUTHI AND SUNDARAVADIVELU: HIGH TEMPERATURE MEASUREMENT USING FIBER BRAGG…

Indian J.Sci.Res. 14 (1): 284-289, 2017

The equation (2) λb is the Bragg wavelength, ξ

is the fiber thermo-optic coefficient, ∆T is temperature

change and ∆λb is the change in the Bragg’s wavelength.

Design

A 2D model of a furnace in rectangle shape is

modeled where the length is 0.6m and breadth 0.2m. The

aluminum material is assigned to the model due to its

high thermal conductivity. The temperature is assigned

at one side of the model which is conducted throughout.

The probe points are placed at three different places and

the temperature is measured.

Similarly a 3D model is designed with the

length 4m, breadth 1m and height 1m. The temperature

is assigned to one face of the cube and the edge probes

are used to measure the temperature.

Experiment To Determine Bragg’s Wavelength Shift

In the model created using Comsol the

temperature is measured using the probe points at

different places. These probe points are replaced by the

FBG sensors measuring the same temperature that was

given in Comsol. If conventional sensors are used at that

points it cannot withstand high temperature.

The Fig. 2 shows the block diagram of FBG

temperature measurement. Broadband light source is

given to the FBG using superluminiscent light emitting

diode(SLED). The light passes via the port 1 and 2 of

circulator to the FBG pasted on the HOTPLATE. All the

connections are made using the fiber pigtails. The FBG

reflects the Bragg’s wavelength which is obtained at the

IMON via port 3of the circulator and then the IMON to

the laptop using USB. The temperature of the hot plate

in increased in steps and the corresponding shift in the

wavelength is noted in IMON software in the laptop.

Figure 2: Block diagram of FBG temperature

measurement.

HIGH TEMPERATURE MEASUREMENT

Simulation of 2D and 3D models

The Fig. 3 shows the 2D model for the

temperature measurement. The temperature is given at

one side of the model which traverses along based on the

conductivity of the material. The blue line on the left

side indicates the temperature assigned. The Fig. 4

shows the time dependent study is done for the model

and the heat is transferred along the model which is

shown in different shades.

Figure 3: The 2D model designed using Comsol

Figure 4: The heat transfer along the model

The Fig. 5 shows the 3D model for the

temperature measurement where the aluminum is

assigned to the entire block and the temperature is

assigned to one face of the block which is indicated with

blue shade. The Fig.6 shows the heat transfer in 3D

model when a time dependent study is performed in

Comsol.

SHRUTHI AND SUNDARAVADIVELU: HIGH TEMPERATURE MEASUREMENT USING FIBER BRAGG…

Indian J.Sci.Res. 14 (1): 284-289, 2017

Figure 5: The 3D model using Comsol

Figure 6: The heat transfer in the 3D model

Sensor Location and Temperature measurement

waveform for 2Dmodel

The sensors are placed at three different places

to measure the heat transfer. The figures at the top of

Fig. 7 shows that the domain probe points placed at the

three different places in the model. The probe points are

used to measure the temperature at different instants.

The bottom part of the figure depicts the respective

waveforms or temperature measured for the probe points

in the figure. Different temperature values like 300 °C or

573.15 K, 500 °C or 773.15 K is given and measured.

Figure 7: Domain probe points placement and

respective temperature (573.15 K) measured

waveforms

The Fig. 8 shows the all the three waveforms of

the probe points plotted in the same the same plot. The

plots here are temperature (K) versus Time (sec).The

blue, green and red lines indicates the waveforms of

first, second and third probe points respectively. The

difference in the waveforms for same temperature

throughout is due to distance from the source of

temperature to the probe points. These probe points are

replaced by the FBG sensors rather than the conventional

sensors.

Figure 8: Waveform for all the three probe points

Sensor Location and Temperature measurement

waveform for 3Dmodel

The Fig. 9 shows the domain boundary probe

points to measure the temperature given at the one face

of the model. The probe is kept at the boundary of the

sides of the 3D model designed.

Figure 9: The domain boundary edge probe points

assignment

SHRUTHI AND SUNDARAVADIVELU: HIGH TEMPERATURE MEASUREMENT USING FIBER BRAGG…

Indian J.Sci.Res. 14 (1): 284-289, 2017

Figure 10: Plot for temperature (573 K)

measurement

The Fig. 10 shows the time dependent study

made for the 3D model and also the waveforms for

temperature measured using the boundary domain probe

points. The temperature of 573 (K) is given and plots are

obtained. The first picture gives the heat transfer in the

3D model where it gradually reaches the end. The

waveforms are temperature (K) versus time (sec).The

two plotted line indicates the measured values at the

instant of the boundary probes. As the heat traverses

through the model gradually, the waveforms obtained is

also different based on the placement of the probes. The

probes at the edge face measure less due to low heat

reached at the end.

EXPERIMENTAL SETUP

The Fig. 11 depicts the practical experimental

setup performed at IIT MADRAS of the FBG senor to

measure the temperature which is given as the

perturbation to the FBG. The temperature to FBG placed

on the ceramic hotplate is measured by increasing in

steps.

Figure 11: Experimental setup performed at IIT

MADRAS

Experimental Results

When the experiment is performed as shown in

the Fig. 11, there is a external perturbation i.e. the

temperature to the FBG given by the ceramic hotplate

over which the FBG is placed. The range of the hotplate

is 40-500 °C. The temperature is increased in the

hotplate in steps and readings are taken. The wavelength

shift in the Bragg’s wavelength is noted in the IMON

evaluation software in the laptop.

The initial reflected Bragg’s wavelength is

1551.54 nm. The Fig. 12 shows the example of reflected

wavelength for temperature of 340 °C that is obtained in

the IMON software and there is a shift noted in the

wavelength which is 1555.0 nm.

Figure 12: Example waveform for 340 °C in

IMON software

These values i.e. the readings obtained for the

different wavelength shift due to temperature change in

the IMON software is tabulated in the excel sheet. For

each temperature increase the readings are tabulated. The

tabulation includes the wavelength shift and the power in

pixel. Then these counts are converted into power in

watts using MATLAB and the plot for power (watts)

versus wavelength (nm).

The Fig. 13 shows the waveform for the initial

reflected Bragg’s wavelength when no temperature is

given to the FBG and it is at 1551.43 nm. Only the input

broadband light source is present in the fiber.

Figure 13: The reflected FBG spectrum

The Fig. 14 depicts the wavelength shift plotted

for 43.5°C from initial 0°C and its shifted to 1551.60

nm. The plot is done for power (watts) versus

wavelength (nm).

SHRUTHI AND SUNDARAVADIVELU: HIGH TEMPERATURE MEASUREMENT USING FIBER BRAGG…

Indian J.Sci.Res. 14 (1): 284-289, 2017

Figure 14: The wavelength shift at 43.5°C

Similarly Fig. 15 depicts the wavelength shift

for 500°C and it is 1557.0 nm. Now from initial reflected

Bragg’s wavelength 1551.43 nm has shifted to 1557.0

nm

Figure 15: The wavelength shift at 500°C

The table 1 shows the tabulated values for

different temperature raised in steps and the respective

wavelength shift of the reflected spectrum that is noted

in the IMON evaluation software and the readings are

also plotted in MATLAB, which is shown in Fig. 15.

Table 1: Tabulation for Temperature and

Wavelength

Temperatu

re(°C)

Wavelengt

h(nm)

Temperatu

re(°C)

Wavelengt

h(nm)

0 1551.43 320 1555.0

108.8 1552.40 370 1555.54

151.2 1552.81 420 1556.06

212.1 1553.50 460 1556.00

270.5 1554.36 500 1557.00

Figure 15: Plot for temperature (°C) versus

wavelength (nm)

From the Fig. 15, it can be inferred that the

wavelength shifts linearly with the temperature as it is

increased. The temperature is increased in steps to obtain

the variations and increase in the reflected Bragg’s

wavelength. The stability of FBG to high temperature

can also be analyzed.

CONCLUSION

A temperature sensor based on Fiber Bragg

gratings is analyzed both theoretically via simulations

and experimentally. The response of the grating to

changes in temperature is very linear and this makes the

FBG a rugged device for sensing applications. Here the

experiment is performed to measure temperature is up to

500°C using the FBG. If the conventional sensors like

resistance temperature detectors ,bimetal switches and

thermistors are used, it will malfunction at

150°C[Fusiek, et.al., 2009] causing instability in the

measurement of the temperature at high sensitive areas

where temperature needs to be monitored regularly. The

fragile nature and its small size with stable

characteristics have helped to sense temperature in

sensitive region. These FBG’s can replace conventional

sensors in remote applications like oil wells, nuclear

reactors, boilers etc...withstanding high temperature

range.

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